1. Field of the Invention
The present invention relates to an acoustic wave transmission system and a method for transmitting an acoustic wave to a drilling metal tubular member, for use in a measurement-while-drilling (MWD) system that can transmit information on a bed or stratum and the condition of drilling equipment while drilling, capable of generating an acoustic wave (or elastic wave) having an amplitude large enough for transmission and a frequency suitable for transmission with a small amount of electric power.
2. Description of the Prior Art
Recent years have seen measurement-while-drilling (MWD) systems that can transmit information on a bed or stratum and the condition of drilling equipment while drilling, by using an acoustic wave propagating through a drill string including a plurality of drilling metal tubular members coupled to one another, such as drill collars and a drill pipe, the MWD systems being intended for reducing the drilling cost and improving the on-the-job safety. There are two types of available MWD systems: mud-pulse systems and electromagnetic-wave systems, which are classified according to which method of transmitting information is used. However, those MWD systems are not good enough to have practical applicability because the transmission rate is limited, the reliability of drilling equipment is decreased, or use environments in which prior art MWD systems can be applied are limited.
MWD techniques for transmitting information using an acoustic wave have captured the spotlight in order to solve the above-mentioned problem. Such MWD techniques can utilize a metal tubular member used for drilling as a medium through which an acoustic wave propagates. Sonic vibration transmission systems using a piezo-electric ceramic as a sonic transmitter have been proposed as one of such MWD techniques. One of such sonic vibration transmission systems is disclosed in, for example, European Pat. Publication No. 0 552 833 A1.
Referring now to FIG. 14, there is illustrated a side view of a prior art acoustic wave transmission system for transmitting an acoustic wave to a drilling metal tubular member, as disclosed in for example Japanese Patent Application Publication (KOKAI) No. 7-294658. FIG. 15 is an exploded perspective view of an oscillator for use in the acoustic wave transmission system, and FIG. 16 is a cross-sectional view of the oscillator of FIG. 15, which is mounted in the acoustic wave transmission system. In the figures, reference numeral 13 denotes a drill collar, 14 denotes a drill pipe, 301 denotes an oscillator for generating an acoustic wave by means of a number of piezo-electric ceramic crystals, 302 denotes a receiver sub, 303 denotes a receiving transducer, 304 denotes an MWD tool, 311 denotes a vibrator comprised of the number of piezo-electric ceramic crystals that are stacked side by side, 312 denotes a coupling block for coupling a metal tubular member with the oscillator 301, and 321 denotes an elastic member such as a plurality of springs. The oscillator 301 is mounted in a recess formed in the drill collar 13. The elastic member 321 forces the body of the oscillator 301 upward in such a manner that the front surface of the coupling block 312 remains engaged against a transverse wall of the drill collar 13.
Referring next to FIG. 17, there is illustrated a diagram showing the waveform of a driving current supplied into the prior art oscillator as shown in FIG. 15. FIG. 18 shows a diagram of the waveform of an acoustic wave generated in the drill collar. An acoustic wave generated by the oscillator 301 enters the drill collar 13 and then propagates upwardly. In the example of FIG. 14, the receiving transducer 303 located above the receiver sub 302 located in the middle of the drill string can receive the acoustic wave. The information can be further transmitted toward the ground through the MWD tool 304 using a prior art MWD method such as the mud-pulse method. In this manner, the acoustic wave generated by the oscillator 301 can be transmitted into the drill collar 13.
When a piezoelectric element is placed in an electric field, it undergoes a strain or distortion the amount of which depends on the magnitude of the electric field. Thus, the application of a voltage across the electrodes sandwiching a piezoelectric element causes a distortion in the piezoelectric element, the amount of distortion corresponding to the voltage. The oscillator 311 mentioned above utilizes this principle. In the oscillator 311, the plurality of piezo-electric elements are stacked side by side and separated by thin electrodes so that a voltage can be applied to each of the plurality of piezo-electric ceramic crystals. A voltage applied to across leads connected to the plurality of thin electrodes produces a driving current 331, as shown in FIG. 17, between any two adjacent electrodes and hence an electric field in each of the plurality of piezo-electric ceramic crystals. The oscillator 311 thus creates sonic vibrations, i.e. an acoustic wave 332 having a frequency corresponding to the frequency of the electric field generated in each of the plurality of piezo-electric ceramic crystals. If the alternating driving current 331 has a frequency equal to a resonance frequency of the oscillator 311, the oscillator 311 vibrates readily at the resonance frequency. The oscillator 311 can thus generate sonic vibrations having large amplitudes, so that the generated acoustic wave 332 can propagate through the drilling string comprised of the plurality of metal tubular members including the drill collar 13 and the drill pipe 14.
Prior art acoustic wave transmission systems for transmitting an acoustic wave into a metal tubular member, which are so constructed as to generate an acoustic wave using an electrostriction effect of each piezo-electric element, have following problems. One problem is that the mechanical strength of each piezo-electric element is relatively low compared with those of metal materials used in the drilling equipment, and there is therefore apprehension that each piezo-electric element becomes damaged because of the impact of drilling and its own electrostriction. Another problem is that since it is difficult to impose an adequate amount of load on the oscillator when mounting it in the drill collar 13, the efficiency of transmitting an acoustic wave generated by the oscillator into a metal tubular member cannot be improved.
A further problem is that because the Curie temperature of a piezo-electric ceramic crystal is about 120.degree. C., for example, and therefore it does not get distorted if its temperature exceeds the Curie temperature, such a piezo-electric ceramic crystal cannot be used in high-temperature environments such as the bottom of a well bore. In addition, the length of the stack of the plurality of piezo-electric elements must be 1 m or more to create sonic vibrations of a low frequency required for transmitting information through a plurality of metal tubular members because the frequency of sonic vibrations is determined according to the thickness of each piezo-electric element. Accordingly, a large amount of energy is needed to drive a large stack of piezo-electric elements, and it is therefore difficult to provide a power supply suitable for supplying adequate power to such a large piezo-electric vibrator intended for systems for transmitting information about the bottom of a borehole. Further, it is therefore difficult to provide a small piezo-electric vibrator suitable for systems for transmitting information about the bottom of a borehole.